sigma (∑) is a string alphabet that consists of a finite set of symbols.
Example ∑ = {a, b} or ∑ = {0,1} or ∑ = {a, b, c}
Here ∑n (for some integer n) denotes the set of strings of length n with symbols from sigma (∑).
In other words,
∑n = {w | w is a string over and | w | = n}.
Hence, for any alphabet, ∑0 denotes the set of all strings of length zero. That is, = {e}.
Let ∑ = {a, b} then
∑0 = {λ}
∑1 = {a, b}
∑2 = (a, b).(a ,b) = aa, ab, ba, bb (4 string with two length over a,b)
So, ∑n = 2n number of string = | ∑n |
∑* :- The set of all strings over an alphabet is denoted by ∑* . It contains set of all the strings that can be generated by iteratively concatenating symbols from any number of times. That is,
Kleene closure example:
Let ∑ = {a,b} then
∑* is a set of all strings generated by a, b. Basically ∑* is a universal set over a,b i.e.
(a + b)*.
Here ∑* = { ε, a, b, aa, ab, ba, bb, aaa, aab, aba, abb, baa, …}
[We can write ε or λ any one]
∑+ :- The set of all strings over an alphabet excluding λ is denoted by ∑+ . That is,
Example:
Let ∑ = {a,b} then
∑+ is a set of all strings generated by a and b with at least one length of the string.
∑+ is also universal set over a,b but excluding i.e. (a + b)+
Here ∑+ = {a, b, aa, ab, ba, bb, aaa, aab, aba, abb, baa, …}
Note : To understand the concept of ∑* and ∑+ is very important for Automata.
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